Both SGML and XML have an internal referencing mechanism that allows links to identified elements within a document. The referenced element must have an attribute that is of type ID (it can have any name but must be defined as having type ID in the schema document). The referring element must have an attribute that is of type IDREF. This type of linking (known as ID-IDREF) is commonly used to create graph like structures within markup (for applications where hierarchies alone are insufficient). An example of ID-IDREF is given below:

    <element id="42">content</element>
    ...
    <referent idref="42"/>

The linking abilities of XML systems are also very powerful. Links can be created both within documents and externally to other documents. Links can be simple or complex, behaviours defined and multiple destinations specified. The linking technology allows users to manage bidirectional and multi-way links, as well as links to a span of text rather than to a single point.

The XML Linking Specification (XLink) and XML Extended Pointer Specification (XPointer) documents contain the details. An XML link can be either a URL or a TEI-style Extended Pointer (XPointer), or both. A URL on its own is assumed to be a resource; if an XPointer or XLink follows it, it is assumed to be a sub-resource of that URL; an XPointer on its own is assumed to apply to the current document (all exactly as with HTML).

An XLink is always preceded by one of #, ?, or |. The # and ? mean the same as in HTML applications; the | means the sub-resource can be found by applying the link to the resource, but the method of doing this is left to the application. An XPointer can only follow a #.